A common way of providing information about a chemical spectrum to a human user is graphically, wherein the ordinate axis is related to the mass properties of a substance being sampled and the abscissa axis is presented as non-negative values, each value being related to the transduced relative concentration of at least one constituent of the substance. One such chemical spectrum is mass spectrometry, whereby the constituents are ionized and the mass/charge ratios of ions are transduced. For example, the spectrum of a pure chemical under near-ideal analytical conditions may have a single non-zero value in the spectrum. In another example, a substance that comprises a plurality of chemical constituents may have a large number of non-zero values in the spectrum.
A common way of using chemical spectra involves signal processing of the spectra. The signal processing can, for example, be intended to remove noise or to detect a relative peak in a chemical spectrum. Signal processing can be used to reduce a multi-valued chemical spectrum to a single value, for example, by detecting a plurality of peaks that correspond to a chemical or a chemical mixture.
In practice, a transduced chemical spectrum of a substance may not correspond to the superposition of the chemical spectra of its constituents. For example, two chemical constituents of a substance may interact so as to enhance or suppress the transduced chemical spectrum of the substance. The processes of sampling the substance and ionizing the substance may further affect the transduced chemical spectrum. It can be appreciated that human interpretation of a graphical presentation of a chemical spectrum is an imperfect art, and that automated interpretation may be a difficult process.
Recent developments in mass spectrometry can provide ensembles of chemical spectra. One such ensemble is a mass-spectrometry image (MSI). One means of transducing such an MSI is by matrix-assisted laser desorption ionization mass spectrometry (MALDI), whereby a substance is spatially sensed in a spatial array. Another means of transducing such an MSI is by desorption electrospray ionization mass spectrometry (DESI), which also spatially senses a substance. An MSI is powerful in providing an estimate of the spatial distribution of chemical constituents of a substance, with the drawbacks of attenuation of the spectra of the constituents. Automated interpretation of an MSI is an active field of research.
Another recent development in mass spectrometry can provide a temporal sequence of chemical spectra. One such sequence is provided by rapid evaporative ionization mass spectrometry (REIMS), whereby a vaporized portion of a substance is sensed by a method that is substantially a form of mass spectrometry, such as is used in DESI. Because the vaporizer can be physically moved over or within a substance, the temporal sequence of chemical spectra can provide an estimate of the spatial distribution of chemical constituents of a substance, with the attending drawbacks.